Since polytetrafluoroethylene (PTFE) has properties such as high chemical resistance and low conductivity, a high melting point and excellent heat resistance, it is used for variety of purposes mainly in chemical and electric fields. It is widely used for machinery purposes such as non-lubricant sliding components taking advantage of the property of a small friction coefficient or surface tension.
On the other hand, PTFE does not dissolve in most solvents except for specific solvents, and has the melting viscosity as high as in a range from 1010 Pa·s to 1011 Pa·s (from 1011 P to 1012 P) at a temperature of 380° C. Thus, various forming methods (such as extrusion and injection molding) used for forming general thermoplastic resin are not easily applied for manufacture of a PTFE product. These forming methods work with resin having melting viscosity upon formation generally in a range from 102 Pa·s to 103 Pa·s.
Conventionally, a method known as sinter forming is a common method of manufacturing a PTFE product. The sinter forming uses powdery PTFE particles as a starting material, and preforms at room temperature (forming aids may be added as needed at this point), and then the formed preform is heated at a temperature equal to or more than the melting point of PTFE (327° C.) for sintering (firing) the whole, and thus a PTFE product is obtained.
Steps for the sinter forming may be determined appropriately in detail according to the form of a product to be obtained, and for example, a PTFE product in a sheet form (a PTFE sheet) may be obtained by forming a cylindrical PTFE product (a PTFE block) by preforming and firing and then by cutting the outer periphery of the formed block (cutting method). Although a sheet with relatively large thickness (for example, not less than 25 μm) may be obtained according to this method, the block size has to be enlarged for manufacturing sheets efficiently. In that case, preforming and firing take a long time (approximately from two to five days, depending on the block size) to inhibit the development of defects such as cracks due to heat distortion. Moreover, sinter forming such as the cutting method is basically a batch process, and thus continuous manufacture of a PTFE product from a starting material is difficult.
Separately from the cutting method, casting also is known as a method of manufacturing a PTFE sheet. By casting, a dispersion of PTFE particles (a PTFE dispersion), which is a starting material, is applied on a support such as a metal plate and then dried and fired, followed by separating from the support for obtaining a PTFE sheet. According to this method, compared to the case of sinter forming, a PTFE sheet is obtained that is thinner and less distorted. The thickness of a sheet obtained by one procedure of application, drying and firing, however, is considered to have an upper limit of about 20 μm for inhibiting minute defects called mud cracks, and a sheet with a thickness above 20 μm has to be obtained by repeating the application and firing of the dispersion a plurality of times. In addition, a product having a form other than a sheet form is difficult to form by casting.
The cutting method, casting and other methods of manufacturing a PTFE product are described in, for example, “Handbook for Fluororesin” (Takaomi Satokawa, ed., published by Nikkan Kogyo Shimbun, Ltd. in 1990 (description for the cutting method on pages 141 to 142, for casting on page 130)).